An accelerometer is a device that senses and measures force or acceleration applied on the accelerometer by monitoring component acceleration vectors, ax, ay, and az, in three orthogonal axes, x, y, and z defining three dimension space. The component vectors ax, ay, and az are respectively indicated by three output signals, typically in voltages, Vx, Vy, and Vz. The signal outputs are given by piezoelectric components in the accelerometer, each monitoring one of the orthogonal axes. Any change in the amount of acceleration experienced by the accelerometer along any of the axes is reflected as a proportional change in the voltage output. The proportion is termed as the “sensitivity” of the accelerometer and is usually expressed in mV/g. Each accelerometer has to be calibrated to determine the sensitivity of each the piezoelectric component monitoring a different axis.
When there is no force acting on a piezoelectric component, such as when the axis monitored by the piezoelectric component is held horizontal to the ground, the piezoelectric component should output no voltage output ideally, indicating that no force is detected (i.e. acceleration equals zero). However, in practice, there is always an offset voltage output from any piezoelectric component. This offset voltage is termed the “zero-g output” and is usually expressed in milli-volts, mV. The residual voltage contributes to a systemic error and must be determined and subtracted from the voltage output of the piezoelectric component before multiplying with the “sensitivity”, in order for the accelerometer readings to be accurate.
Therefore, an accelerometer has to be calibrated to determine and correct these errors before use.
U.S. Pat. No. 6,810,738, Sakaguchi discloses a method of calibrating a 3-axis accelerometer by holding the accelerometer still at two precise orientations, such that the monitored axes are in specific angles convenient to apply trigonometric solutions to determine the values of zero-g signal output and the sensitivity for each axis.
There are several disadvantages with this method. Firstly, factory conditions are not the same as field conditions and thus, the factory calibration is at best a good estimate of the field conditions, and is subject to inaccuracy or deviation due to environmental factors such as temperature and humidity changes. Furthermore, this method does not address production variations within the accelerometer, as such whether the angles between the axes defined by the piezoelectric components are inaccurately set. Furthermore, as the sensitivity and zero-g output of an accelerometer could vary due to environmental variations and gradual component degradation, calibration of the accelerometer should be done from time to time but this is not possible using Sakaguchi's method which requires high precision factory equipment to determine the angular orientation to gravity.
Therefore, it is desirable to provide a method of calibrating or improving the accuracy of the accelerometer which can be used in the field by a user, and without the need of precision equipment.